I work for the Northwest Center for Alternatives to Pesticides (NCAP) based in Eugene. We are updating our resource page on grasshoppers, and we have info on the page about using a jar with water and molasses buried in the ground so that grasshoppers will fall in and not be able to get out. I'm not sure where this information came from, and if it's really an effective way to trap grasshoppers? Do you know, or do you have other suggestions for DIY grasshopper traps for home gardeners?
Molasses has been shown to be an attractant and feeding stimulant for a couple of species of North American grasshoppers. However, I can find no instances of research, where molasses has been tested as a bait for a grasshopper trap. Instead, scientists have looked at whether or not molasses might be a bait that could be used to deliver insecticides to grasshoppers, by integrating the insecticide into a molasses formulation.
Based upon my reading of the literature, I don't think that molasses would make a good bait for a grasshopper trap. First, since it is an attractant, molasses could potentially attract MORE grasshoppers to a home garden ~ which would be counterproductive for grasshopper control. Second, molasses was less attractive to grasshoppers than fatty-acid based baits. Thus, I could imagine a situation where molasses might be attractive enough to get grasshoppers into an area, but not attractive enough to draw them to a trap. Third, I would be concerned that the molasses might attract small mammals or rodents who might be caught in the trap.
I would defer to the UC IPM guidelines for grasshopper control. In particular, their notation about landscape planning and design, and using a green border as a 'trap crop' seems promising. Individual plants can also be netted for protection, if grasshopper populations are not too high.